Microporous polyolefin membrane, and method of producing the same

ABSTRACT

The present invention provides a microporous polyolefin membrane of high permeability and novel structure, and also provides a method of producing the same, wherein its average pore size is gradually decreases from at least one membrane surface towards its center. The method of producing the microporous polyolefin membrane comprises the steps of extruding the solution, composed of 10 to 50 weight % of (A) a polyolefin having a weight-average molecular weight of 5×105 or more or (B) a composition containing this polyolefin and 50 to 90 weight % of a solvent, into a gel-like formed article and removing the solvent therefrom, wherein a treatment step with a hot solvent is incorporated.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of prior application Ser.No. 10/649,732, filed on Aug. 28, 2003 now U.S. Pat. No. 7,479,243,which was a Divisional Application of prior application Ser. No.09/806,308, filed on Aug. 20, 2001, now U.S. Pat. No. 6,666,969, whichwas a §371 National Stage Application of PCT/JP99/05345, filed on Sep.29, 1999, the previous applications being hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a microporous membrane composed of anultra-high-molecular-weight polyolefin, more particularly to amicroporous polyolefin membrane of high permeability, and a method ofproducing the same.

2. Description of the Prior Art

Microporous membranes are widely used in various applications such asbattery separators (in particular, lithium ion type primary andsecondary batteries), large-sized battery separators or electrolyticcapacitor separators for electric cars or the like, various types ofseparation membranes (e.g., for water treatment, ultrafiltration,microfiltration and reverse osmosis), various types of filters,moisture-permeable and waterproof clothes and the base materialsthereof, etc.

Microporous polyolefin membranes are produced, e.g., by an extractionprocess comprising the steps of mixing a polyolefin with an organicsolvent and inorganic powder such as finely powdered silica, melting andmolding the mixture, and extracting the organic solvent and inorganicpowder. This method needs a process of extracting the inorganic powder,and permeability of the membrane produced depends largely on particlesize of the inorganic powder used and is difficult to control.

Recently, various processes have been proposed to produce high-strength,microporous membranes which contain an ultra-high-molecular-weightpolyolefin. For example, Japanese Patent Laid-Open Nos. 60-242035,61-195132, 61-195133, 63-39602, 63-273651, 3-64334, and 3-105851disclose processes to produce microporous membranes by forming agel-like sheet from a heated solution of a polyolefin compositioncontaining an ultra-high-molecular-weight polyolefin dissolved in asolvent, stretching it while heating, and removing the solvent byextraction. These processes give the microporous polyolefin membranescharacterized by a narrow pore size distribution and small pore size,which are used for battery separators or the like.

Recently, lithium ion type secondary batteries are required to have aproperty of high energy density, and microporous membranes of polyolefinhaving a shut-down function are widely used for the battery separators,to meet the increasing demands for the batteries. However, for thespecial battery purposes, e.g., those requiring high output at lowtemperature, a microporous polyolefin membrane of small pore size maycause problems, e.g., increased inner resistance of the battery.Therefore, the microporous polyolefin membranes high in safety and ionpermeability are in demand. It is also necessary to easily control sizeof the pores, both for those in the vicinity of the membrane surface andits inside.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a microporouspolyolefin membrane of high permeability and novel structure. It isanother object of the present invention to provide a method of producingthe same.

The inventors of the present invention have found, after havingextensively studied to solve the above problems, that a microporouspolyolefin membrane having a pore size distribution in the thicknessdirection can be produced by treating, with a hot solvent, the gel-likeformed article or stretched product thereof obtained by extruding asolution of ultra-high-molecular-weight polyolefin or compositioncontaining an ultra-high-molecular-weight polyolefin dissolved in asolvent, or the microporous membrane obtained by removing the solventfrom the gel-like formed article or stretched product thereof, to reachthe present invention.

The present invention provides a microporous polyolefin membrane,composed of (A) a polyolefin having a weight-average molecular weight of5×10⁵ or more or (B) a composition containing this polyolefin, whereinaverage pore size is gradually decreases from at least one membranesurface towards its center. The present invention also provides a methodof producing the same microporous polyolefin membrane, comprising thesteps of extruding the solution, composed of 10 to 50 weight % of (A) apolyolefin having a weight-average molecular weight of 5×10⁵ or more or(B) a composition containing this polyolefin and 50 to 90 weight % of asolvent, into a gel-like formed article and removing the solventtherefrom, wherein a treatment step with a hot solvent is incorporated.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 presents the transmission electron microgram (magnification:2,500) which shows the sectional structure of the microporous polyolefinmembrane.

FIG. 2 presents the transmission electron microgram (magnification:2,500) which shows the sectional structure of the microporous polyolefinmembrane having pores of uniform size throughout the body of themembrane, corresponding to the one prepared in a COMPARATIVE EXAMPLE.

DETAILED DESCRIPTION OF THE INVENTION

1. Polyolefin

The polyolefin (A) for the microporous polyolefin membrane of thepresent invention has a weight-average molecular weight of 5×10⁵ ormore, preferably in a range from 1×10⁶ to 15×10⁶. When theweight-average molecular weight is less than 5×10⁵, the membrane tendsto suffer deteriorated strength.

The polyolefin composition (B) contains at least 1 weight % of anultra-high-molecular-weight polyolefin having a weight-average molecularweight of 5×10⁵ or more, preferably 7×10⁵ or more, more preferably in arange from 1×10⁶ to 15×10⁶. When the ultra-high-molecular-weightpolyolefin content is less than 1 weight %, the microporous polyolefinmembrane of high strength may not be obtained, because of lack of themolecular chains of the ultra-high-molecular-weight polyolefin entwiningwith each other. Other polyolefins than the ultra-high-molecular-weightpolyolefin in the polyolefin composition, i.e., those having aweight-average molecular weight of less than 5×10⁶, preferably have alower weight-average molecular weight limit of 1×10⁴. When a polyolefinhaving a weight-average molecular weight of less than 1×10⁴ is used,rupture is likely to take place in the membrane, failing to provide adesired microporous membrane.

When a mixture of (B-1) an ultra-high-molecular-weight polyolefin havinga weight-average molecular weight of 5×10⁵ or more and (B-2) apolyolefin having a weight-average molecular weight of less than 5×10⁵is used, the (B-2)/(B-1) weight ratio of the composition is 0.2 to 20,preferably 0.5 to 10. When the (B-2)/(B-1) weight ratio is below 0.2,the gel-like formed article produced tends to shrink in the thicknessdirection and decrease in permeability, and viscosity of the solutionincreases to deteriorate its molding processability. When the CB-2)/(−1)weight ratio is above 20, on the other hand, concentration of theultra-high-molecular-weight polyolefin is excessively low, to result indensified gel structure and lowered permeability of the microporousmembrane produced.

Examples of the above polyolefins include crystalline homopolymers,two-stage polymers or copolymers of ethylene, propylene, 1-butene,4-methyl-pentene-1 or 1-hexene, or blends thereof. Preferable among themare polypropylene, polyethylene and compositions thereof.

The above polyolefin or polyolefin composition has a molecular weightdistribution (weight-average molecular weight/number-average molecularweight) of 300 or less, preferably 5 to 50. The molecular weightdistribution of above 300 is undesirable, because the membranecomprising such a composition may suffer breakdown by the lowermolecular weight components to lose its strength as a whole. When thepolyolefin composition is used, it can be obtained by mixing anultra-high-molecular-weight polyolefin having a weight-average molecularweight of 5×10⁵ or more and a polyolefin having a weight-averagemolecular weight of less than 5×10⁵ with each other in an adequate ratioto secure the molecular weight distribution to fall in the above range.The polyolefin composition may be the one produced by the multi-stagepolymerization or containing two or more types of polyolefins, so longas it has a molecular weight and molecular weight distribution fallingin the above ranges.

When the porous polyolefin membrane of the present invention is used fora lithium battery separator or the like, the polyolefin therefor may beincorporated with a polymer capable of giving a shut-down property atlow temperature. These polymers include low-density polyethylene,low-molecular-weight polyethylene and linear ethylene-α-olefincopolymer.

The low-density polyethylenes useful for the present invention includebranched polyethylene (LDPE) produced by the high pressure process, andlinear low-density polyethylene (LLDPE) produced by the low pressureprocess. The LDPE normally has a density of around 0.91 to 0.93 g/cm³,and melt index (MI at 190° C. and 2.16 kg load) of 0.1 to 20 g/10minutes, preferably 0.5 to 10 g/10 minutes. The LLDPE normally has adensity of around 0.91 to 0.93 g/cm³, and melt index (MI at 190° C. and2.16 kg load) of 0.1 to 25 g/10 minutes, preferably 0.5 to 10 g/10minutes. The preferable composition, when the low-density polyethyleneis included, is composed of 1 to 69 weight % of anultra-high-molecular-weight polyethylene having a weight-averagemolecular weight of 7×10⁵, 98 to 1 weight % of the high-densitypolyethylene, and 1 to 30 weight % of the low-density polyethylene.

The low-molecular-weight polyethylene useful for the present inventionis a polyethylene of low degree of polymerization, having a molecularweight of 1,000 to 4,000 and melting point of 80 to 130° C., i.e., DSCpeak temperature determined in accordance with JIS K7121, and preferablyof polyethylene wax having a density of 0.92 to 0.97 g/cm³. Thelow-molecular-weight polyethylene can be incorporated with (A) apolyolefin or (B) a polyolefin composition at 1 weight % or more,preferably 10 to 70 weight %.

The linear ethylene-α-olefin copolymer useful for the present inventionas the one capable of giving a shut-down property at low temperatureinclude a linear ethylene-α-olefin copolymer produced in the presence ofa single-site catalyst, e.g., metallocene catalyst, e.g.,ethylene-butene-1 copolymer, ethylene-hexene-1 copolymer orethylene-octene-1 copolymer. The ethylene-α-olefin copolymer has amelting point (DSC peak temperature determined in accordance with JISK7121) of 95 to 125° C., preferably 100 to 120° C. When a copolymerhaving a melting point of less than 95° C. is used, the battery producedwill have characteristics significantly deteriorated at hightemperature. Use of a copolymer having a melting point more than 125° C.is also undesirable, because the shut-down property may not be exhibitedat an adequate temperature level. The ethylene-α-olefin copolymer has aweight-average molecular weight/number-average molecular weight ratio(Mw/Mn ratio, Q value) of 1.5 to 3.0, preferably 1.5 to 2.5. Whenincorporated with the ethylene-α-olefin copolymer, the polyethylene orits composition can give the microporous polyethylene membrane for alithium battery separator or the like, which can be shut down at lowtemperature when battery inside temperature increases as a result ofelectrode short circuit. Moreover, it can drastically improve dependenceof membrane resistance on temperature when the separator is shut down,and allow the shut-down temperature to be freely controlled. Thepolyethylene or its composition is incorporated with theethylene-α-olefin copolymer at 2 to 80 weight %, preferably 5 to 50weight %. At below 2 weight %, the rapid shut-down effect may not beexpected at low temperature, and at above 80 weight %, the microporouspolyethylene membrane will significantly lose strength.

When a polyethylene is used as the polyolefin for the present invention,it may be incorporated with polypropylene to provide microscopicroughness on the microporous membrane surface, in order to help themembrane hold the electrolytic solution. The polypropylene compoundsuseful for the present invention include homopolypropylene having aweight-average molecular weight of 1.0×10⁴ or more, preferably in arange from 3.0×10⁴ to 1.0×10⁶, and ethylene/propylene random copolymerand ethylene/propylene block copolymer containing ethylene at 1.0 weight%. When polypropylene having a weight-average molecular weight of lessthan 1.0×10⁴ is used, the microporous polyolefin membrane produced willbe difficult to bore. When the ethylene content exceeds 1.0 weight %,the polyolefin has deteriorated crystallinity, making the microporouspolyolefin membrane it gives difficult to bore.

The polypropylene content is 5 to 30 weight %, based on the polyolefinor its composition, preferably 5 to 25 weight %. At below 5 weight %, itcannot form a number of evenly dispersed irregularities, making littlecontribution to the improved electrolyte-holding effect. At above 30weight %, on the other hand, the microporous polyolefin membraneproduced will have significantly deteriorated strength. At a stillhigher content, it will be difficult to form the sheet, because of thephase separation between the polyethylene and polypropylene taking placeduring the sheet-forming step.

Incidentally, the polyolefin composition containing theultra-high-molecular-weight polyolefin component may be incorporatedwith various additives such as antioxidants, ultraviolet absorbers,antiblocking agents, pigments, dyes, inorganic fillers, etc., ifrequired, within limits not harmful to the object of the presentinvention.

2. Microporous Polyolefin Membrane

The microporous polyolefin membrane of the present invention hasmicropores whose average size is gradually decreases from at least onemembrane surface towards its center, and has high permeability. Forexample, referring to FIG. 1, which presents the transmission electronmicrogram (magnification: 2,500) showing the sectional structure of themicroporous polyolefin membrane, average size of the pores is larger inthe vicinity of the membrane surface than in the vicinity of the center.

Each of the through-holes in the microporous polyolefin membrane of thepresent invention may have a large opening and smaller size at thecenter, or have a large opening and tapers off towards the oppositeside.

The microporous polyolefin membrane of the present invention has a novelstructure with a layer of smaller pores to sustain strength of themembrane, and larger openings in the vicinity of the surface to securehigh permeability.

When the membrane has a thickness of (d), the average pore size (a) inthe area from the surface to a depth of d/16 is 0.05 to 50 μm,preferably 1 to 30 μm, and the average pore size (b) in the other areais 0.01 to 30 μm, preferably 0.03 to 2 μm, where the relationship(b)<(a) should hold.

The microporous polyolefin membrane of the present invention,characterized by the above structure, has an overall porosity of 35 to95%, preferably 45 to 80%, and permeability of 5 to 500 sec/100 cc,preferably 5 to 250 sec/100 cc.

3. Method of Producing the Microporous Polyolefin Membrane

The microporous polyolefin membrane of the present invention can beobtained by the method comprising the steps of extruding the solution ofthe above-described polyolefin or polyolefin composition dissolved in asolvent into a gel-like formed article, removing the solvent, with orwithout stretching, and drying the formed article, wherein a treatmentstep with a hot solvent is incorporated, as described earlier.

(1) Method of Producing the Microporous Membrane

In the method of the present invention as the basis for producing amicroporous membrane, a solution is prepared by dissolving the abovepolyolefin or its composition in a solvent while heating. Examples ofthe solvents include aliphatic or cyclic hydrocarbons such as nonane,decane, decalin, p-xylene, undecane, dodecane, paraffin oils, etc., andfractions of mineral oils having boiling points substantially equal tothose of the above hydrocarbons. These solvents have a viscosity of 30to 500 cSt at 25° C., preferably 50 to 200 cSt. Uneven extrusion resultswhen viscosity is below 30 cSt at 25° C., making it difficult to kneadthe solution, whereas the solvent is difficult to remove in the posttreatment when viscosity is above 500 cSt.

Dissolution of the polyolefin or its composition while heating iscarried out by stirring its solution at a temperature at which it iscompletely dissolved in the solvent, or uniformly mixing it and solventwith each other in an extruder. When the polyolefin or its compositionis to be dissolved in the solvent with stirring, the dissolvingtemperature varies depending on the types of polymers and solvents used.It is in the range of 140 to 250° C. in the case of polyethylenecomposition. It is preferable to effect the dissolution in an extruder,when a high-concentration solution of the polyolefin or its compositionis used to produce a microporous membrane.

When the dissolution is to be effected in an extruder, the polyolefin orits composition is first charged in the extruder to be molten, at atemperature preferably 30 to 100° C. above melting point of thepolyolefin, although varying depending on type of the polyolefin used.The melting point is described later. For example, it is 160 to 230° C.,preferably 170 to 200° C., in the case of polyethylene, and 190 to 270°C., preferably 190 to 250° C., in the case of polypropylene. Then; aliquid solvent is added to the polyolefin or its composition in themiddle of the extruder.

The concentration of the polyolefin or its composition is 10 to 50weight % based on the total solution, preferably 10 to 30 weight %, orconversely the concentration of the solvent is 90 to 50 weight %,preferably 90 to 70 weight %. When the polyolefin or its compositionconcentration is less than 10 weight % (or the solvent concentration ismore than 90 weight %), swelling and neck-in are likely to take place atthe exit of a die in the process of forming sheets. Accordingly, it isdifficult to keep good molding processability and self-supportingcharacteristics of the sheet produced. On the other hand, when thepolyolefin or its composition concentration is more than 50 weight % (orthe solvent concentration is less than 50 weight %), the sheet producedwill suffer various problems, such as excessive shrinkage in thethickness direction, lowered porosity, and deteriorated moldingprocessability. Accordingly, it is difficult to produce the microporousmembrane having large pores. It is possible to control permeability ofthe membrane by changing the polyolefin or its composition concentrationin the above range.

Next, the heated solution of the molten and kneaded polyolefin or itscomposition is extruded through a die or the like, in such a way tosecure the final membrane thickness of 5 to 250 μm, either directly orvia another extruder.

Usually used as a die is a sheet die having a rectangular orifice, but adouble-pipe hollow die, an inflation die, etc. may be used. When thesheet die is used, a die gap is usually 0.1 to 5 mm, and heated at 140to 250° C. in the extrusion process. In this case, a casting roll speedis usually 20-30 cm/minute to 15 m/minute.

The solution extruded through the die is formed into a gel-like articleby quenching. It is cooled to 90° C. or below, preferably to 80 to 30°C., at a speed of at least 50° C./minute, by cooling the die or thegel-like sheet. As a method of cooling the gel-like sheet, directcontact with cooling air, cooling water or other cooling media, contactwith a roll cooled by a coolant, etc. may be employed. Contact with acooled roll is a preferable method.

The gel-like article produced will have a rough higher-order structure,and the pseudo-cell units that constitute the structure also becomerough at an excessively low cooling speed, whereas dense cellar unitsresult at a sufficiently high cooling speed. Decreasing cooling speed tobelow 50° C./minute makes the gel structure close to that comprisingindividual bubbles, and increases its crystallization degree, making itdifficult to remove the solvent.

The cooling roll is preferably kept at 30° C. to polyolefincrystallization temperature, preferably 40 to 90° C. When cooling rolltemperature is excessively high, the gel-like sheet will be cooledslowly to sufficiently increase thickness of the walls defining thelamella structure of the polyolefin, of which the gel structure iscomposed, tending to make the micropores into the individual bubbles. Asa result, the solvent becomes difficult to remove, and membranepermeability decreases. When it is excessively low, on the other hand,the gel-like sheet is quenched excessively rapidly. As a result, the gelstructure is excessively densified, decreasing pore size andpermeability. The gel-like sheet is withdrawn at 1 to 20 m/minute,preferably 3 to 10 m/minute. Low withdrawal rate is preferable, becauseneck-in tends to take place on the sheet, making it excessivelystretchable.

The gel-like article is then stretched, as required, by an ordinarymethod, such as a tenter, roll, calender method or a combination thereofat a given stretching ratio. It may be monoaxial or biaxial. In the caseof biaxial stretching, the gel-like article may be stretched in thelongitudinal and transverse directions simultaneously or sequentially.The simultaneous stretching is more preferable.

The stretching temperature should be in a range from the polyolefincrystal dispersion temperature to 10° C. above the crystal meltingpoint, preferably in a range from the crystal dispersion temperature tothe crystal melting point. For a polyethylene composition containing anultra-high-molecular-weight polyethylene, for example, it is preferablyin a range from 90 to 140° C., more preferably from 100 to 130° C. Ifthe stretching temperature is higher than the melting point plus 10° C.,the molecular orientation by stretching does not take place because theresin melts. If the stretching temperature is lower than the crystaldispersion temperature, on the other hand, the resin is softenedinsufficiently, with the result that the membrane tends to break andstretching ratio cannot be well controlled.

The crystal dispersion temperature is the temperature level determinedby measuring the temperature characteristics of the dynamicviscoelasticity, in accordance with ASTM D4065, and melting point is thepeak temperature observed by a DSC, in accordance with JIS K7211 (thesame in the following pages).

The stretching ratio is not limited, but preferably 2 to 400 times asareal stretching ratio, more preferably 15 to 400.

The article, stretched or not stretched, is then washed with a solventto remove the residual solvent. Solvents used for this solvent-removingtreatment may be volatile ones, including hydrocarbons such as pentane,hexane and heptane; chlorinated hydrocarbons such as methylene chlorideand carbon tetrachloride; fluorinated hydrocarbons such astrifluoroethane; and ethers such as diethyl ether and dioxane. Thesevolatile solvents may be used individually or in combination, and theirselection depends on the types of the nonvolatile solvents used todissolve the polyolefin composition. Washing methods with the solventsinclude an extraction method with solvent, a method of spraying solventor a combination thereof.

The washing of the thermally set article with a solvent should beperformed to such an extent that the content of the residual solvent isless than 1 weight %. The article is finally dried to remove the washingsolvent by a heating method, an air-drying method, etc.

(2) Step of Treatment with Hot Solvent

The microporous polyolefin membrane of the present invention having anovel structure can be produced by incorporating a step of treatmentwith hot solvent.

This step of treatment with hot solvent may be incorporated before orafter the above-described solvent removal step. The formed article maybe treated either directly or indirectly, wherein it is directly broughtinto contact with the hot solvent in the former, and heated after beingbrought into contact with the solvent in the latter (they arehereinafter referred to as the direct and indirect method,respectively). In other words, any method can be adopted, so long as theformed article can be brought into contact with the hot solvent in anystage.

Those falling into the category of direct method include dipping thearticle in the hot solvent, spraying the hot solvent onto the article,and spreading the hot solvent over the article. Of these, the dippingmethod is more preferable because it can treat the article moreuniformly. Those falling into the category of indirect method includedipping the article in the solvent, spraying the solvent onto thearticle, and spreading the solvent over the article, the article thustreated being further treated with the hot solvent by hot rolling,heating in an oven or dipping in the hot solvent.

The solvent temperature is from the crystal dispersion temperature tomelting point plus 10° C. of the (A) polyolefin or (B) polyolefincomposition or more, but at melting point of the (A) polyolefin or (B)polyolefin composition. When the polyolefin is a polyethylene, it ispreferably 110 to 130° C., more preferably 115 to 125° C. At below thecrystal dispersion temperature, the effects of treatment with the hotsolvent and improving permeability are little expected. At above themelting point plus 10° C., on the other hand, the undesirable effectsmay result, to cause rapid loss of strength or breaking of themicroporous membrane. The treatment time is preferably 0.1 seconds to 10minutes, more preferably 5 seconds to 1 minute. When treatment time isless than 0.1 seconds, the effects of treatment with the hot solvent andimproving permeability are little expected. When it exceeds 10 minutes,on the other hand, the undesirable effects may result, to cause rapidloss of strength or breaking of the microporous membrane.

The solvent useful for the above treatment may be the same as ordifferent from the one used for producing the above-described polyolefinsolution. One of the most desirable solvents is liquid paraffin.

The microporous polyolefin membrane of the present invention, producedby the above-described procedure to have the above-described structure,is a highly permeable membrane, having a permeability of 5 to 500sec/100 cc, preferably 5 to 250 sec/100 cc, overall porosity of 35 to95%, preferably 45 to 80%, and average pore size of 0.05 to 50 μm atleast in the vicinity of one of the surfaces.

Another advantage of the present invention is controllability of poresize and porosity of the membrane only by changing temperature and timeof the hot solvent treatment step, without changing the overall process(refer to EXAMPLE 5, described later).

The resulting microporous polyolefin membrane is, if necessary,subjected to a hydrophilic treatment by plasma irradiation, impregnationwith a surface active agent, surface grafting, etc.

EXAMPLES

The present invention is described in more detail by the followingpreferred embodiments, which by no means limit the present invention.The properties cited in the preferred embodiments were determined by thefollowing test methods:

(1) Weight-average molecular weight and molecular weight distribution:Determined by gel permeation chromatography (GPC), with GPC analyzer(Waters), column (Tosoh's GMH-6) and o-dichlorobenzene as the solvent,operated at 135° C. and flow rate of 1.0 ml/minute.(2) Membrane thickness: Determined by a tracer type thickness meter(Mitsutoyo Litematic).(3) Air permeability: Measured according to JIS P8117.(4) Porosity: Determined by the weight method.(5) Tensile strength: Breaking strength of the 10 mm wide specimen stripwas determined according to ASTM D822.(6) Average pore size: Determined by measuring sizes of 100 pores,selected from the transmission electron microgram of the porous membranesection, to find the average.(7) Shut-down temperature: Determined by heating the membrane to a giventemperature and measuring the temperature level at which itspermeability increases to 100,000 seconds/100 cc or more.

Example 1

A polyethylene composition (melting point: 135° C., crystal dispersiontemperature: 90° C.) having an Mw/Mn ratio of 14, composed of 20 weight% of an ultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.5×10⁶ and 80 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.0×10⁵ was incorporated with 0.08 weight parts of aphenol-based antioxidant and 0.08 weight parts of phosphorus-basedantioxidant, all based on 100 weight parts of the polyethylenecomposition, to prepare a polyethylene composition. Twenty (20) weightparts of the polyethylene composition was fed to a biaxial extruder (58mm in diameter, L/D ratio=42, high kneading type segment), and 80 weightparts of liquid paraffin was also fed to the biaxial extruder from theside feeder, to prepare a polyethylene solution in the extruder bymelting the polyethylene composition and kneading it with the liquidparaffin at 200° C. and 200 rpm. The resulting solution was extrudedfrom the T-die attached at the extruder end at 190° C., and taken up bya cooling roll with a die-roll space of 20 mm, to prepare a gel-likesheet. The sheet was then biaxially-stretched at 115° C. at an arealstretching ratio of 5 by 5, to prepare the stretched sheet. Theresulting stretched membrane was washed with an excessive quantity ofhexane to remove the residual liquid paraffin, and dried andheat-treated to prepare a microporous polyethylene membrane.

The resulting membrane was set on a 10 cm-square metallic frame, dippedin a liquid paraffin bath kept at 120° C. for 5 seconds, dipped in anexcessive quantity of hexane bath to wash out the liquid paraffin, anddried at room temperature and then in an air oven at 115° C. for 2minutes. The microporous polyolefin membrane thus prepared had athickness of 32 μm, porosity of 67%, permeability of 65 seconds andtensile strength of 700 kgf/cm². The membrane section had the structuresimilar to that shown in FIG. 1 which presents the electron transmissionmicrogram, and an average pore size of 8.2 μm in the vicinity of the onesurface, 0.5 μm in the vicinity of the other surface, and 0.12 μm atnear the center. The results are given in Table 1.

Example 2

A polyethylene composition (melting point: 135° C., crystal dispersiontemperature: 90° C.) having an Mw/Mn ratio of 14, composed of 20 weight% of an ultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.5×10⁶ and 80 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.0×10⁵ was incorporated with 0.08 weight parts of aphenol-based antioxidant and 0.08 weight parts of phosphorus-basedantioxidant, all based on 100 weight parts of the polyethylenecomposition, to prepare a polyethylene composition. Twenty (20) weightparts of the polyethylene composition was fed to a biaxial extruder (58mm in diameter, L/D ratio=42, high kneading type segment), and 80 weightparts of liquid paraffin was also fed to the biaxial extruder from theside feeder, to prepare a polyethylene solution in the extruder bymelting the polyethylene composition and kneading it with the liquidparaffin at 200° C. and 200 rpm. The resulting solution was extrudedfrom the T-die attached at the extruder end at 190° C., and taken up bya cooling roll with a die-roll space of 20 mm, to prepare a gel-likesheet. The sheet was then biaxially stretched at 115° C. at an arealstretching ratio of 5 by 5, to prepare the stretched sheet. Theresulting stretched membrane was washed with an excessive quantity ofhexane to remove the residual liquid paraffin, and dried andheat-treated to prepare a microporous polyethylene membrane.

The resulting membrane was set on a 10 cm-square metallic frame, dippedin a liquid paraffin bath kept at 118° C. for 10 seconds, dipped in anexcessive quantity of hexane bath to wash out the liquid paraffin, anddried at room temperature and then in an air oven at 115° C. for 2minutes. The microporous polyolefin membrane thus prepared had athickness of 30 μm, porosity of 58%, permeability of 200 seconds andtensile strength of 650 kgf/cm². The membrane section had the structuresimilar to that shown in FIG. 1 which presents the electron transmissionmicrogram, and an average pore size smaller at the membrane center thanin the vicinity of the surface. The results are given in Table 1.

Comparative Example 1

A microporous membrane was prepared in the same manner as in EXAMPLE 1except that it was not treated with the hot solvent. The properties ofthe membrane are shown in Table 1. The microporous membrane surface andsection had the structures similar to that shown in FIG. 2 whichpresents the electron transmission microgram, with uniform pore sizethroughout the membrane. It had a shut-down temperature of 135° C.

Comparative Example 2

The properties of a commercial microporous membrane, manufactured byCelgard, are shown in Table 1. The microporous membrane surface andsection, observed by a scanning electron microscope and transmissionelectron microscope, respectively, had the structures with uniform poresize throughout the membrane.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 2Film forming conditions Stretching conditions Stretching ratio 5 × 5 5 ×5 5 × 5 manufactured by Temperature (° C.) 115 115 115 CelgardConditions of treating with hot solvent after making after stretching —— membrane Temperature (° C.) 120 118 — — Properties of microporousmembranes Thickness (μm) 32 20 25 25 Air permeability (sec/100 cc) 65200 650 550 Porosity (%) 67 58 41 40 Tensile strength (kgf/cm²):TD 700650 805 460 Average pore size 8.2 15.0 0.03 0.08 [front surface - d/16](mm) Average pore size 0.5 9.5 0.03 0.08 [back surface - d/16] (mm)Average pore size 0.12 0.5 0.03 0.08 [middle section] (mm) [Frontsurface - d/16]: Average pore size in the area from the one surface to adepth of d/16, wherein (d) is thickness of the membrane [Back surface -d/16]: Average pore size in the area from the other surface to a depthof d/16, wherein (d) is thickness of the membrane (it should be notedthat the [surface - d/16] is a positive number) [Middle section]:Average pore size in the area other than the [front surface - d/16] and[back surface - d/16] areas.

Example 3

A polyethylene composition (melting point: 135° C., crystal dispersiontemperature: 90° C.) having an Mw/Mn ratio of 14, composed of 20 weight% of an ultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.5×10⁶ and 80 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.0×10⁵ was incorporated with 0.08 weight parts of aphenol-based antioxidant and 0.08 weight parts of phosphorus-basedantioxidant, all based on 100 weight parts of the polyethylenecomposition, to prepare a polyethylene composition. Twenty (20) weightparts of the polyethylene composition was fed to a biaxial extruder (58mm in diameter, L/D ratio=42, high kneading type segment), and 80 weightparts of liquid paraffin was also fed to the biaxial extruder from theside feeder, to prepare a polyethylene solution in the extruder bymelting the polyethylene composition and kneading it with the liquidparaffin at 200° C. and 200 rpm. The resulting solution was extrudedfrom the T-die attached at the extruder end at 190° C., and taken up bya cooling roll with a die-roll space of 20 mm, to prepare a gel-likesheet. The sheet was then biaxially stretched at 115° C. at an arealstretching ratio of 5 by 5, to prepare the stretched sheet. Theresulting stretched membrane was washed with an excessive quantity ofhexane to remove the residual liquid paraffin, and dried andheat-treated to prepare a microporous polyethylene membrane.

The resulting membrane was dipped in a liquid paraffin bath kept at 118°C. for 10 seconds while it was held at both ends, dipped in an excessivequantity of hexane bath to wash out the liquid paraffin, and dried atroom temperature and then in an air oven at 115° C. for 2 minutes. Themicroporous polyolefin membrane thus prepared had a thickness of 30 μm,porosity of 58%, permeability of 200 seconds and tensile strength of 650kgf/cm² in the TD direction. Pore size in each layer, found by thescanning electron microgram for the surface and transmission electronmicrogram (FIG. 1) for the section, is given in Table 2.

Example 4

A polyethylene composition (melting point: 135° C., crystal dispersiontemperature: 90° C.) having an Mw/Mn ratio of 14, composed of 20 weight% of an ultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.5×10⁶ and 80 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.0×10⁵ was incorporated with 0.08 weight parts of aphenol-based antioxidant and 0.08 weight parts of phosphorus-basedantioxidant, all based on 100 weight parts of the polyethylenecomposition, to prepare a polyethylene composition. Twenty (20) weightparts of the polyethylene composition was fed to a biaxial extruder (58mm in diameter, L/D ratio=42, high kneading type segment), and 80 weightparts of liquid paraffin was also fed to the biaxial extruder from theside feeder, to prepare a polyethylene solution in the extruder bymelting the polyethylene composition and kneading it with the liquidparaffin at 200° C. and 200 rpm. The resulting solution was extrudedfrom the T-die attached at the extruder end at 190° C., and taken up bya cooling roll with a die-roll space of 20 mm, to prepare a gel-likesheet. The sheet was then biaxially stretched at 115° C. at an arealstretching ratio of 5 by 5, to prepare the stretched sheet. Theresulting stretched membrane was washed with an excessive quantity ofhexane to remove the residual liquid paraffin, and dried andheat-treated to prepare a microporous polyethylene membrane.

The microporous membrane thus prepared had a thickness of 25 μm,porosity of 40%, permeability of 550 seconds and tensile strength of 805kgf/cm² in the TD direction. The resulting membrane was set on a 10cm-square metallic frame, onto which an excessive quantity of liquidparaffin was sprayed, brought into contact with a hot roll kept at 118°C. for 10 seconds, dipped in an excessive quantity of hexane bath towash out the liquid paraffin, and dried at room temperature and then inan air oven at 115° C. for 2 minutes. The microporous polyolefinmembrane thus prepared had a thickness of 30 μm, porosity of 55%,permeability of 200 seconds and tensile strength of 700 kgf/cm². Poresize in each layer, found by the scanning electron microgram for thesurface and transmission electron microgram (FIG. 1) for the section, isgiven in Table 2.

Example 5

A microporous membrane was prepared in the same manner as in EXAMPLE 3except that it was treated with the hot solvent at 122° C. for 10seconds. The results are given in Table 2. As shown, pore size in themicroporous membrane can be changed by changing treatment temperatureand time with the hot solvent.

Example 6

A microporous membrane was prepared in the same manner as in EXAMPLE 3except that it was treated with the hot solvent at 118° C. for 2seconds. The results are given in Table 2.

Example 7

A polyethylene composition (melting point: 135° C., crystal dispersiontemperature: 90° C.), composed of 20 weight % of anultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.0×10⁶, 66.7 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.9×10⁵ and 13.3 weight % of a low-density polyethylene havinga melt index of 2.0 (190° C., 2.16 kg) was incorporated with 0.375weight parts of an antioxidant based on 100 weight parts of thepolyethylene composition, to prepare a polyethylene composition. Fifteen(15) weight parts of the polyethylene composition was fed to a biaxialextruder (58 mm in diameter, L/D ratio=42, high kneading type), and 85weight parts of liquid paraffin was also fed to the biaxial extruderfrom the side feeder, to prepare a polyethylene solution in the extruderby melting the polyethylene composition and kneading it with the liquidparaffin at 200° C. and 200 rpm. The resulting solution was extrudedfrom the T-die attached at the extruder end in such a way to have 30 μmas thickness of the final product, and taken up by a cooling roll, keptat 50° C., to prepare a gel-like sheet. The sheet was then biaxiallystretched at 115° C. at an areal stretching ratio of 5 by 5, to preparethe stretched sheet. The resulting membrane was set on a 10 cm-squaremetallic frame, dipped in a liquid paraffin bath kept at 115° C. for 10seconds, dipped in an excessive quantity of hexane bath to wash out theliquid paraffin, and dried at room temperature and then in an air ovenat 115° C. for 1 minute. Table 2 gives the thickness, porosity,permeability, tensile strength, and pore size in each layer of theresulting microporous polyethylene membrane.

Example 8

A polyethylene composition (melting point: 135° C., crystal dispersiontemperature: 90° C.). composed of 20 weight % of anultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.0×10⁶ and 80 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.3×10⁵ was incorporated with 0.375 weight parts of anantioxidant based on 100 weight parts of the polyethylene composition,to prepare a polyethylene composition. A mixture composed of 30 weightparts of the polyethylene composition and 5 weight parts of polyethylenewax (Mitsui Highwax-100, melting point: 115° C., molecular weight:1,000, Mitsui Chemicals) was fed to a biaxial extruder (58 mm indiameter, L/D ratio=42, high kneading type), and 70 weight parts ofliquid paraffin was also fed to the biaxial extruder from the sidefeeder, to prepare a polyethylene solution in the extruder by meltingthe polyethylene composition and kneading it with the liquid paraffin at190° C. and 200 rpm. The resulting solution was extruded from the T-dieattached at the extruder end in such a way to have 30 μm as thickness ofthe final product, and taken up by a cooling roll, kept at 50° C., toprepare a gel-like sheet. The sheet was then biaxially stretched at 115°C. at an areal stretching ratio of 5 by 5, to prepare the stretchedsheet. The resulting membrane was set on a 10 cm-square metallic frame,dipped in a liquid paraffin bath kept at 115° C. for 10 seconds, dippedin an excessive quantity of hexane bath to wash out the liquid paraffin,and dried at room temperature and then in an air oven at 115° C. for 1minute. Table 2 gives the thickness, porosity, permeability, tensilestrength, and pore size in each layer of the resulting microporouspolyethylene membrane.

Example 9

A polyethylene composition (melting point: 165° C., crystal dispersiontemperature: 90° C.), composed of 20 weight % of anultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.5×10⁶, 60 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.5×10⁵ and 20 weight % of a polyethylene having aweight-average molecular weight of 5.1×10⁵ was incorporated with 0.375weight parts of an antioxidant based on 100 weight parts of thepolyethylene composition, to prepare a polyethylene composition. Thirty(30) weight parts of the polyethylene composition was fed to a biaxialextruder (58 mm in diameter, L/D ratio=42, high kneading type), and 70weight parts of liquid paraffin was also fed to the biaxial extruderfrom the side feeder, to prepare a polyethylene solution in the extruderby melting the polyethylene composition and kneading it with the liquidparaffin at 200 rpm.

The resulting solution was extruded at 190° C. from the T-die attachedat the extruder end, and taken up by a cooling roll to prepare agel-like sheet. The sheet was then biaxially stretched at 115° C. at anareal stretching ratio of 5 by 5, to prepare the stretched sheet. Theresulting membrane was set on a 10 cm-square metallic frame, dipped in aliquid paraffin bath kept at 115° C. for 10 seconds, dipped in anexcessive quantity of hexane bath to wash out the liquid paraffin, anddried at room temperature and then in an air oven at 115° C. for 1minute. Table 2 gives the thickness, porosity, permeability, tensilestrength, and pore size in each layer of the resulting microporouspolyethylene membrane.

Example 10

A polyethylene composition (melting point: 135° C., crystal dispersiontemperature: 90° C.), composed of 17.6 weight % of anultra-high-molecular-weight polyethylene (UHMWPE) having aweight-average molecular weight of 2.5×10⁶, 70.8 weight % of ahigh-density polyethylene (HDPE) having a weight-average molecularweight of 3.3×10⁵ and 11.6 weight % of an ethylene-α-olefin copolymer(density: 0.915, melting point: 108° C., ethylene-octene-1 copolymer,Affinity FM1570, the Dow Chemical) produced in the presence of asingle-site catalyst was incorporated with 0.375 weight parts of anantioxidant based on 100 weight parts of the polyethylene composition,to prepare a polyethylene composition. Thirty (30) weight parts of thepolyethylene composition was fed to a biaxial extruder (58 mm indiameter, L/D ratio=42, high kneading type), and 70 weight parts ofliquid paraffin was also fed to the biaxial extruder from the sidefeeder, to prepare a polyethylene solution in the extruder by meltingthe polyethylene composition and kneading it with the liquid paraffin at200 rpm.

The resulting solution was extruded at 190° C. from the T-die attachedat the extruder end, and taken up by a cooling roll to prepare agel-like sheet. The sheet was then biaxially stretched at 115° C. at anareal stretching ratio of 5 by 5, to prepare the stretched sheet. Theresulting membrane was set on a 10 cm-square metallic frame, dipped in aliquid paraffin bath kept at 115° C. for 10 seconds, dipped in anexcessive quantity of hexane bath to wash out the liquid paraffin, anddried at room temperature and then in an air oven at 115° C. for 1minute. Table 2 gives the thickness, porosity, permeability, tensilestrength, and pore size in each layer of the resulting microporouspolyethylene membrane. Its shut-down temperature was measured at 105°C., indicating that it was improved in safety for a battery separator.

Example 11

A microporous membrane was prepared in the same manner as in EXAMPLE 4except that it was not stretched and treated with the hot solvent at118° C. for 2 seconds. Table 2 gives the thickness, porosity,permeability, tensile strength, and pore size in each layer of theresulting microporous polyethylene membrane.

Comparative Example 3

A microporous membrane was prepared in the same manner as in EXAMPLE 3except that it was not treated with the hot solvent. Table 2 gives thethickness, porosity, permeability, tensile strength, and pore size ineach layer of the resulting microporous polyethylene membrane.

Comparative Example 4

A microporous membrane was prepared in the same manner as in EXAMPLE 3except that it was neither stretched nor treated with the hot solvent.Table 2 gives the thickness, porosity, permeability, tensile strength,and pore size in each layer of the resulting microporous polyethylenemembrane.

TABLE 2 Comparative Example Example 3 4 5 6 7 8 9 10 11 3 4 polyolefincomposition (A) (A) (A) (A) (B) (C) (D) (E) (A) (A) (A) Stretchingconditions 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 — 5 × 5 —Stretehing ratio 115 115 115 115 115 115 115 115 — 115 — Temperature (°C.) — — — Thermal setting before after before before before beforebefore before before — — (before or after solvent removal) Temperature(° C.) 118 118 122 118 115 115 115 115 118 — — Time (seconds) 10 10 1010 10 10 10 10 10 10 — Properties of microporous membranes Thickness(μm) 30 30 28 33 30 30 30 30 45 30 45 Air permeability (sec/100 cc) 200200 36 150 102 114 61 78 13 550 28 Porosity (%) 58 55 76 50 59 61 66 6582 40 72 Tensile strength (kgf/cm²):TD 650 700 620 680 680 680 760 680100 805 160 Average pore size 15.0 8.2 15.0 15.0 12.8 11.2 15.0 12.316.2 0.03 0.21 [front surface - d/16] (mm) Average pore size 9.5 0.514.8 9.5 8.7 8.8 7.5 9.3 9.3 0.03 0.18 [back surface - d/16] (mm)Average pore size 0.5 0.12 14.8 0.35 0.47 0.48 0.5 0.5 0.20 0.03 0.18[middle section] (mm) polyolefin composition; (A): 2.5 × 10⁶UHMWPE(30 wt%)/6.8 × 10⁵HDPE(70 wt %) (B): 2.5 × 10⁶UHMWPE(20 wt %)/3.9 ×10⁵HDPE(66.7 wt %)/LDPE(13.3 wt %) (C): 2.5 × 10⁶UHMWPE(19 wt %)/3.3 ×10⁵HDPE(76 wt %)/PE Wax(5 wt %) (D): 2.5 × 10⁶UHMWPE(20 wt %)/3.3 ×10⁵HDPE(60 wt %)/PP(20 wt %) (E): 2.5 × 10⁶UHMWPE(17.6 wt %)/3.3 ×10⁵HDPE(70.8 wt %)/ethylene-octene-1 copolymer(11.6 wt %) [Frontsurface - d/16]: Average pore size in time area from the one surface toa depth of d/16, wherein (d) is thickness of the membrane [Backsurface - d/16]: Average pore size in time area from the other surfaceto a depth of d/16, wherein (d) is thickness of the membrane (it shouldbe noted that the [surface - d/16] is a positive number) [Middlesection]: Average pore size in the area other than the [front surface -d/16] and [back surface - d/16] areas.

As described in detail, the microporous polyolefin membrane of thepresent invention is a highly permeable microporous membrane having anovel structure with the larger pores in the vicinity of at least onemembrane surface and smaller pores inside in the thickness direction,the pore size decreasing in the thickness direction. This structuremakes the membrane suitable for various devices, e.g., batteryseparators and liquid filters. The production method of the presentinvention is also effective in that it gives the microporous membranehaving a structure with large pores inside in the thickness direction,without changing the production process.

1. A battery separator which uses a microporous polyolefin membranecomposed of (A) a polyolefin having a weight-average molecular weight of5×10⁵ or more or (B) a composition containing a polyolefin having aweight-average molecular weight of 5×10⁵ or more, wherein average poresize of the microporous polyolefin membrane gradually decreases from atleast one membrane surface towards its center.
 2. A battery which uses,for its separator, a microporous polyolefin membrane composed of (A) apolyolefin having a weight-average molecular weight of 5×10⁵ or more or(B) a composition containing a polyolefin having a weight-averagemolecular weight of 5×10⁵ or more, wherein average pore size of themicroporous polyolefin membrane gradually decreases from at least onemembrane surface towards its center.
 3. The battery separator of claim1, wherein the microporous polyolefin membrane is composed of acomposition containing a polyolefin having a weight-average molecularweight of 5×10⁵ or more and at least 1 weight percent of anultra-high-molecular-weight polyolefin having a weight-average molecularweight of 1×10⁶ to 15×10⁶.
 4. The battery separator of claim 1, whereinthe microporous polyolefin membrane with the average pore size thatgradually decreases from the at least one membrane surface towards itscenter is obtained by treating a gel-like article with an organicsolvent having a temperature of 110 to 130° C.
 5. The battery separatorof claim 3, wherein the microporous polyolefin membrane with the averagepore size that gradually decreases from the at least one membranesurface towards its center is obtained by treating a gel-like articlewith an organic solvent having a temperature ranging from a crystaldispersion temperature to a melting point plus 10° C. of the polyolefinhaving the weight-average molecular weight of 5×10⁵ or more or theultra-high-molecular-weight polyolefin having a weight-average molecularweight of 1×10⁶ to 15×10⁶.
 6. The battery of claim 2, wherein themicroporous polyolefin membrane is composed of a composition containinga polyolefin having a weight-average molecular weight of 5×10⁵ or moreand at least 1 weight percent of an ultra-high-molecular-weightpolyolefin having a weight-average molecular weight of 1×10⁶ to 15×10⁶.7. The battery of claim 2, wherein the microporous polyolefin membranewith the average pore size that gradually decreases from the at leastone membrane surface towards its center is obtained by treating agel-like article with an organic solvent having a temperature of 110 to130° C.
 8. The battery of claim 6, wherein the microporous polyolefinmembrane with the average pore size that gradually decreases from the atleast one membrane surface towards its center is obtained by treating agel-like article with an organic solvent having a temperature rangingfrom a crystal dispersion temperature to a melting point plus 10° C. ofthe polyolefin having the weight-average molecular weight of 5×10⁵ ormore or the ultra-high-molecular-weight polyolefin having aweight-average molecular weight of 1×10⁶ to 15×10⁶.